The need for the controlled and sustained release of an active agent into the environment exists in many fields, for instance, in residential and other consumer fields, in agriculture, and in telecommunications and other areas of technology, and the prior art knows a variety of methods for achieving such release.
One class of methods comprises dispersion of the active agent throughout (or dissolution in) a substantially inert matrix from which the active agent is gradually released into the environment. The discussion herein will be limited to controlled-release methods of this type, and to devices using this method. Such devices are frequently referred to as "monolithic" devices.
Matrices that have been used in monolithic devices include plasticized polyvinylchloride, rubbers, (see, for instance, U.S. Pat. No. 3,318,269) and some polyurethanes. Prior art rubber matrices typically are cured by conventional rubber chemistry, e.g., by sulfur vulcanization or by means of peroxides. See, for instance, U.S. Pat. No. 3,639,583. These curing processes typically comprise a high-temperature step, i.e., a process step during which the matrix material with the active agent therein is heated to a temperature above about 100.degree. C., frequently even to a substantially higher temperature.
A prior art example of the use of a polyurethane matrix can be found in U.S. Pat. No. 4,189,467 ('467), incorporated herein by reference, which discloses a plastic composition comprising polyurethane. The matrices disclosed in the above patent typically are relatively nonelastomeric, due to the inclusion of filler materials, use of low molecular weight (30-400) reactive materials (e.g., butane diol) and/or to the choice of low molecular weight polyols and low molecular weight isocyanates. For instance, in an exemplary composition the patent discloses the use of about 43 parts of filler material, with the polyurethane formed by the reaction of relatively low molecular weight polyols and isocyanates, resulting in a relatively high modulus polyurethane requiring the use of a plasticizer to reduce the stiffness of the composition. A further possible reason for the frequent use of a plasticizer in the prior art composition is the facilitation of the migration of the active agent through the relatively impermeable matrix to the release surface.
U.S. Pat. No. 4,094,970 discloses that elastomeric materials, specifically, matrices formed from a polyisocyanate and units of a long chain polyol (having a molecular weight from 1000 to 10,000) and at least one of a short chain polyol and/or polyamine (having a molecular weight of 62 to 1,000), can advantageously be used as matrices for insecticidal active compounds. Such mixtures typically require high isocyanate content, resulting in a relatively stiff matrix. Use of polyamines in high isocyanate formulations further increases the stiffness of the matrix. Furthermore, such formulations tend to have high exotherms during reaction.
Many of the prior art controlled release systems of the type discussed herein require a high temperature processing step. Such a step frequently is a complicating factor when active agents having relatively high vapor pressure, or active agents that are subject to deactivation at the elevated temperature, are to be incorporated into the matrix material.
Some prior art controlled release systems require the use of a carrier substance, frequently a plasticizer, to at least assist in the transport of the active agent to the release surface. Such systems are often subject to deposition of active agent on the release surface (blooming). However, blooming can also be due to the properties of the active agent, and may be desired at times. Use of a carrier substance may also lead to incomplete release of active agent. The latter shortcoming is typically due to an increase in the glass transition temperature of the matrix with increasing plasticizer loss, resulting in a decreasing release rate and eventual virtual cessation of release, frequently with a substantial amount of active agent still present in the matrix. Such behavior leads to unnecessarily shortened product life.
Because of the importance of controlled release of active agents, a controlled release system that does not have some of the shortcomings of prior art systems, e.g., that is free of high temperature reaction or processing steps, that is resistant to hydrolysis and water uptake, is not subject to syneresis, that has the potential for release of a high proportion of the active agent, and that furthermore has wide applicability, would be of substantial benefit. This application discloses such a system.